Our Nervous System 433 



The Spinal Cord. The spinal cord is an organ about eighteen 

 inches long and about one-half an inch in diameter, composed of 

 nerve tissue. It lies in the tubular cavity formed by the bony 

 rings of the backbone which protect it from injury. It connects 

 with the brain at the base of the skull, and extends downward 

 almost to the level of the lowest ribs. Like the brain it is com- 

 posed of gray and of white nerve tissue, but unlike the brain the 

 white tissue is on the outside and the gray tissue underneath. The 

 cord is smooth and not of uniform size in its whole length. 

 It gives off thirty-one pairs of nerves through openings between 

 the rings of the backbone. These nerves function when directed 

 to act by the brain. For instance, when a person desires to run, 

 his brain sends an order to the spinal nerve cells and they in turn 

 transmit the order over the nerves to the muscles used in running. 

 In this way all orders for voluntary motion are given by the 

 brain. 



Reflex Action. Reflex action is a term used to denote the 

 power of the spinal cord to give directions for action with- 

 out the aid of the brain. The purpose of reflex action is two- 

 fold, to protect the parts of the body from injury, and to relieve 

 the brain of work by controlling important processes of the 

 body. 



The way in which reflex action protects the body is illustrated 

 when you quickly withdraw your hand from a hot iron with 

 which it has come in contact. Before the sensation of heat 

 reaches the brain, the spinal cord has already directed the muscles 

 of your hand to withdraw it, thus protecting it from further 

 possible injury. 



The way in which reflex action relieves the work of the 

 brain may be illustrated by reference to the digestive and other 

 vital processes of the body. All of these are controlled by the 

 spinal cord. They are said to be carried on unconsciously, which 

 means that the brain does not give them attention. 



All habitual movements of the body are governed by the spinal 

 cord. They are reflex actions. When first used, the brain cells 

 give the orders to the spinal cord to act, but gradually the spinal 

 cord assumes control, thus relieving the brain and leaving it free 



